1. Field of the Invention
The present invention relates to a molecular fluorine laser system including line-narrowing elements and method for generating a VUV laser beam having a spectral linewidth of less than substantially 1 pm.
2. Description of the Related Art
Vacuum-UV microlithography takes advantage of the short wavelength of the molecular fluorine laser (157.6 nm), which allows the formation of structures of 0.1 xcexcm or below by photolithographic exposure on semiconductor substrates. TFT annealing and micro-machining applications may also be performed advantageously at this wavelength.
Given the limited choice of high quality optical materials available in this wavelength range for manufacturing imaging lenses, requirements of minimal chromatic aberrations restrict spectral linewidths of the laser source for refractive and partially achromatic imaging systems to below 1 pm. The expectation is that spectral linewidths be between 0.1 pm and 0.2 pm, and perhaps even below 0.1 pm in the future. Conventional molecular fluorine lasers emit VUV beams having spectral linewidths of greater than 1 pm.
A disadvantage of narrowing of spectral linewidth in a laser is that it commonly leads to a significant decrease of efficiency and output power. Therefore, it is recognized in the present invention that to achieve a desired high throughput for 157 nm wafer steppers or wafer scanners, it would be advantageous to have a line-narrowed molecular fluorine laser emitting an output beam of less than 1 pm, with a high output power that averages anywhere from several watts to more than 10 watts.
It is therefore a first object of the present invention to provide a VUV laser system having a narrow linewidth, i.e., less than substantially 1 pm for producing small structures on silicon wafers.
It is a second object of the invention to provide a VUV laser having a linewidth of 1 pm or less which exhibits sufficient output power, i.e., at least several Watts, to allow high throughput for VUV lithography applications at 157 nm.
Methods and apparatuses are provided in accord with the above objects, such as a narrow band molecular fluorine laser system including an oscillator and an amplifier, wherein the oscillator produces a 157 nm beam having a linewidth less than 1 pm and the amplifier increases the power of the beam above a predetermined amount, such as more than one or several Watts. The oscillator includes a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, electrodes within the discharge chamber connected to a discharge circuit for energizing the molecular fluorine, and a resonator including the discharge chamber and line-narrowing optics for generating the laser beam having a wavelength around 157 nm and a linewidth less than 1 pm.
The amplifier preferably comprises a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, electrodes connected to the same or a similar discharge circuit, e.g., using an electrical delay circuit, for energizing the molecular fluorine. The amplifier discharge is timed to be at or near a maximum in discharge current when the pulse from the oscillator reaches the amplifier discharge chamber.
The line-narrowing optics preferably include one or more etalons tuned for maximum transmissivity of a selected portion of the spectral distribution of the beam, and for relatively low transmissivity of outer portions of the spectral distribution of the beam. A prism beam expander is preferably provided before the etalons for expanding the beam incident on the etalon or etalons. Two etalons may be used and tuned such that only a single interference order is selected.
The line-narrowing optics may further include a grating for selecting a single interference order of the etalon or etalons corresponding to the selected portion of the spectral distribution of the beam. The resonator further preferably includes an aperture within the resonator, and particularly between the discharge chamber and the beam expander. A second aperture may be provided on the other side of the discharge chamber.
The line-narrowing optics may include no etalon. For example, the line optics may instead include only a beam expander and a diffraction grating. The beam expander preferably includes two, three or four VUV transparent prisms before the grating. The grating preferably has a highly reflective surface for serving as a resonator reflector in addition to its role of dispersing the beam.
The line-narrowing optics may include an etalon output coupler tuned for maximum reflectivity of a selected portion of the spectral distribution of the beam, and for relatively low reflectivity of outer portions of the spectral distribution of the beam. This system would also include optics such as a grating, dispersive prism or etalon, preferably following a beam expander, for selecting a single interference order of the etalon output coupler. The resonator would preferably have one or more apertures for reducing stray light and divergence within the resonator.
In any of above configurations including a grating, a highly reflective mirror may be disposed after the grating such that the grating and HR mirror form a Littman configuration. Alternatively, the grating may serve to retroreflect as well as to dispserse the beam in a Littrow configuration. A transmission grating or grism may also be used.
The buffer gas preferably includes neon and/or helium for pressurizing the gas mixture sufficiently to increase the output energy for a given input energy and to increase the energy stability, gas and tube lifetime, and/or pulse duration. The laser system further preferably includes a gas supply system for transferring molecular fluorine into discharge chamber and thereby replenishing the molecular fluorine, therein, and a processor cooperating with the gas supply system to control the molecular fluorine concentration within the discharge chamber to maintain the molecular fluorine concentration within a predetermined range of optimum performance of the laser.
The laser system may also include a spectral filter between the oscillator and the amplifier for further narrowing the linewidth of the output beam of the oscillator. The spectral filter may include an etalon or etalons following a beam expander. Alternatively, the spectral filter may include a grating for dispersing and narrowing the beam. In the grating embodiment, the spectral filter may include a lens focusing the beam through a slit and onto a collimating optic prior to impinging upon the beam expander-grating combination.